In recent years, development of lasers is remarkable. In particular, with respect to solid lasers or semiconductor lasers having an emitting region in near infrared to infrared wavelengths, high-output and small-sized products have become easily available. These lasers are very useful as exposure sources during direct plate making from digital data of computers, etc.
Positive working lithographic printing plate materials for infrared laser contain an alkaline aqueous solution-soluble binder resin and an infrared absorbing dye (IR dye) that absorbs light to generate a heat as essential components. In unexposed areas (image areas), the IR dye acts as a dissolution inhibitor to substantially lower dissolution of the binder resin due to a mutual action with the binder resin, whereas in exposed areas (non-image areas), it is dissolved in an alkaline developing solution because its mutual action with the binder resin is weakened due to the generated heat, thereby forming a lithographic printing plate.
However, in such positive working lithographic printing plate precursors for infrared laser, it cannot be said that under various conditions of use, a difference between dissolution resistance of unexposed areas (image areas) to developing solutions and dissolution of exposed areas (non-image areas) in developing solutions is sufficient, and there was involved a problem such that excessive development or development failure likely occurs due to changes in conditions of use. Further, since image forming ability of lithographic printing plate relies upon heat generation of infrared laser exposure on the recording layer surface, there was involved another problem such that in the vicinity of a support, image formation is insufficient due to diffusion of the heat, i.e., the amount of heat to be used for solubilizing the recording layer becomes low, whereby a difference between exposed areas and unexposed areas becomes small, leading to insufficient reproducibility of highlights.
For example, development latitude was not substantially problematic in positive working lithographic printing plate materials undergoing plate making by UV exposure, i.e., conventional lithographic printing plate materials containing an alkaline aqueous solution-soluble binder resin and an onium salt or a quinonediazide compound and having a function such that in unexposed areas (image areas), the onium slat or quinonediazide compound acts as dissolution inhibitor due to a mutual action with the binder resin, whereas in exposed areas (non-image areas), it is decomposed by light to generate an acid and act as a dissolution accelerator.
On the other hand, in positive working lithographic printing plate materials for infrared laser, an infrared absorber acts only as a dissolution inhibitor of unexposed areas (image areas) but does not accelerate dissolution of exposed areas (non-image areas). Accordingly, in positive working lithographic printing plate materials, for ensuring sufficient sensitivity, it is necessary to previously use binder resins having high dissolution in alkaline developing solutions such that dissolution of the exposed areas is high. As a result, dissolution of unexposed areas also becomes high. Accordingly, when the surface is rubbed to form scuffs, dissolution resistance is poor so that scars are visualized as film diminishment.
On the other hand, for suppressing film diminishment due to scuffs on the surface, it is necessary to take measures for lowering dissolution of the photosensitive layer, leading to reduction of the sensitivity. Accordingly, there is a problem such that sensitivity and scuffing resistance are inconsistent with each other.
In addition, for solving such problems, JP-A-10-250255 discloses that a thermosensitive layer whose change in dissolution during image formation is large is provided in an upper layer, whereas a layer having high alkali dissolution is provided in a lower layer. According to the technology of JP-A-10-250255, an improving effect is found, but consistence between the sensitivity and the scuffing resistance does not reach a satisfactory level yet.
On the other hand, with respect to multilayer positive working lithographic printing plate materials for infrared laser, JP-A-11-218914 discloses a multilayer photosensitive image forming material utilizing a binder having a specific structure in a lower layer. However, JP-A-11-218914 does not mention scuffing resistance at all. Further, U.S. Pat. No. 6,242,156 discloses a lithographic printing plate with low tackiness and excellent block resistance, containing a radiation-sensitive layer having a roughness (Ra), caused by unevennesses of a support, of 0.2 microns or more. However, U.S. Pat. No. 6,242,156 is concerned with a so-called non-processing type and does not describe any working examples regarding a thermal positive working type.
Further, it is disclosed in JP-A-11-218914 to provide an infrared laser image forming material for direct plate making having good development latitude, in which a photosensitive layer has a two-layer structure, and low image forming property is improved by a recording layer having a devised alkaline aqueous solution-soluble high-molecular compound in a lower layer thereof. However, improvement of the alkali-soluble resin in the lower layer is insufficient. With respect to the image forming property, improvement of the upper layer portion is necessary.
Moreover, European Patent No. 997,272 discloses a multilayer positive working lithographic printing plate material for infrared laser containing a block copolymer in an upper layer.